Automatic frequency control system



y i, 1951 E. DURAND EIAL AUTOMATIC FREQUENCY CONTROL SYSTEM Filed Oct. sf, 1945 ATTORNEY A BEDSEQJ, Rd ONT m m MUW a 9 mmm RL EL H ozfiwn m wwm P v m m & a $45? II ozmew mozwmw. .5 5&8 mm $525535 W B Q Nam h W Y 532; "I H1? 3f 5 23 ma 9 565 I a a $2M N x H B n i. m II n 03" a .w I J m m S M 2 Patented July 31, 195i "r,

QUENCY CONTROL YSTEM Eric Durand and Lowell Hollingsworth, Cam- AUTOMATIC FRE bridge, Mass., assignors, by mesne assignments,

to the United States of America as represented by the Secretary of War Application October 3, 1945, SerialNo. 620,132 Claims, (01, 2'50 36) This invention relates ingeneral to electrical circuits and, more specificallyto a system for automatic frequency stabilization of oscillators :and similar apparatus.

. In many types of radio object locating devices it is necessaryto control the frequency of local oscillators so that the intermediate frequency, resulting from mixing the local oscillator with the signal, will be constant. The frequency of the local oscillator tends to, change because of varia- -tions in the load, in the temperature of the appa- ,ratus,;and other, factors. It issometimes desir- -;abl e, tochangerthefrequency of the signal, thereby necessitating a change in frequency of the local oscillator sothat the intermediate frequency will remain the same as before.

The change in local oscillator frequency must he a rapid one, as in many radio object locating devices the signal consists ofrecurring pulses of extremely: short duration, When the local oscillator is producing such a frequency that the in- .termediate frequency is not the desired frequency, a complete pulse or chain of pulses may he lost by attenuation unless an immediate ..,change of 1 frequency of the local oscillator is am d V Tt is an object of this invention to provide a .means which will automatically stabilize the frequency of a local oscillator. I a

A second object is to provide a means for stabiliz'ing an I.- F. voltage. Y

Another objectis to provide a means such that the device fails to control the local oscillator frequency the control will be established.

. It is a further object of this invention te m-0 .vide a: system whichwill control the local oscillator frequency in accordance with either the sum or difference sideband frequency at the option of the user to insure the best operation of the local oscillator when using the device to control an intermediate frequency. a

, Generally, this invention consists of a discriminator connected to a trigger circuit. The output I voltage of the trigger circuit is positive or negative depending upon which tube of the trigger circuit is conducting. This voltage is used to :changethe frequency of the oscillator. During normal operation, with control established, the trigger circuit alternates rapidly between its two gst'able conditions. If, however, control is lost, Ithe output voltage will be constant and after a (specified time, a relay circuit operates a motor to again restore the'oscillator to the proper frefuency, at which time the control circuit resumes 19 mma positive voltage and is also connected through Other objects, features and advantages of this invention will suggest themselves to those skilled in the part and Will become apparent from the following description of the invention taken in connection with the accompanying drawings in which: v I 1 l I r Fig. 1 is a circuit diagram embodying theprinciples of this inyention; and q Fig. 2 is a graph which represents the voltage output of the frequency discriminator with respect to the frequency of the local oscillator,

' Referring now more particularly to Fig. 1, the mixer l l is connected to terminal In of the primary of transformer [2; Th secondary of transformer I2 is connected across capacitor 1| sand to the plates of diodes l4 and I5. Terminal i0 is also connected through capacitor I6 to a center tap of the secondary of transformer-I2 and to one side of choke l1. The cathode of diodes l4 and I5 are connected together by the primary of transformer I8. Resistors land- Z0 are connected in series across the-primary of transformer 18. Capacitors 2| and 22 are-also connected in series across th primary of transformer Ill. The junction of resistors I9 and. 20 is connected to the junction of capacitors 2! and 22 and then to choke l1. 7

The secondary of transformer I 8, the center point of which is grounded, is connected be- The junction of reof tubes 23 and 24 are connected together and 'through resistor 21 to ground. Two terminals of the primary sides of transformers 28 and-Z9 are joined and supplied by a positive voltage.

A second terminal, of the primary winding 9f transformer 28 is connected to the plateof tuhe 23. Similarly theremaining terminal of theprimary winding of transformer 2945" connected to the plate of tube 24. Across the secondary of transformer 28 is resistor 30. Similarly across the secondary of transformer 29 is resistor 59. On side of resistor 30 and resistor 60 is connected to ground. The other side of resistor 30 is connected to the cathode of-tubet I, Similarly the ungrounded terminal of resistor efl is connected to the cathode oftube 32.. The plate of tube 3| is connected through resistor 33 to resistor 34, which .is shuntedby capacitor 35 to the grid of tube3. 2. A further connection from the plate of tube.;3l is made to potentiometer and 31." The, plate of tube 32 I is connected to strut 62.

age produced by the dis'crin'iinator. "of said amplifier is applied in a push-pulrmanfner to the cathode of the'tri gg'ef'circuittubes-3| and 32 by the secondary'windings of transformers through resistor 38 to a positive voltage, and is also connected through resistor 39, which is shunted by capacitor 40, to the grid of tube 3|. A further connection is made from the plate of tube 32 to potentiometer 4|. The grid of tube 3| is connectedfthrough resistor 42 to a'negative voltage and the grid of tube '32 is connected through resistor 43 to a negative voltage. A negative voltage is also applied to one terminal of potentiometers 36, 31, and ;4|. The grid of tube 32 is also connected to terminal 58 of motor-driven cam relay 44. In certain positions of the cam of relay 44 the grid of tube 32 will be grounded by coming in contact with terminal59.

The variable tap of potentiometer -41 is connected to the grid of tube 45 and through capacitor 46 to ground. The variable tap of Ipotentiometer 31 is connected to the grid of tube 41 and through capacitor 48 toground. The cathodes of tubes 45 and 41 are grounded. The plates of tubes @45 and "41 are connected together andthrough the winding of relay 49 --to a p'ositive voltage. The variable tap of potentiometer '36 is connected to "terminal 55 'on relay l4. Output-terminal '50 is connected to terminal 56 on relay '44. Terminal 51 of relay 44 is connected to a voltage whose magnitude may be variable. An A.-C. motor is mechanically connected to relay 44. One terininal of relay 49 is connected to the A.-C. motor. The remaining terminal of relay 49 "is connected to alternating voltage supply. The second ter- -minal of an alternating voltage'supply is connected directlyto the A.-C. mo'torwinding.

"Output terminal 50 is connected to grid 52 which controls the current flow from cathode 6| Strut 62 is connected to the cavity resonator of os'ci-llator'tu'be 53'. Strut 621s heated by the current passing through-it and isarranged to' e'xpand or contract with changes intempe-ra- 'tu're. Expansionor contraction of' strut 62 alters the cavity resonator and changes'the frequency of theoscilla-tor.

In operation, the I.-F. voltage whose frequency is to be controlled is appl-ied to terminal ill by mixer ThisL-F. voltage is obtained by-heterodyning' a radio frequency voltage with the -output of the local oscillator 53, which is connected to mixer "H.

I Diodes 1-4 and I5, operating in conjunction with the connected apparatus, produce a voltage output in transformer 18, thepolarity ii-rid "magnitude of which isa'f'u'nction of the frequency of the incoming voltage in relation to the the "Radio Engineer's Handbook by F. E. Terfman, published by McGraw-Hill Book Co mpany, ineqin 1943.

The output oftrans'former' It is applied to the rid of tubes 23 and 24. 'Tubes 23 "and 24 ar used as a push-pull amplifier to amplify the volt- The output 28 and 29. The trigger'circuit, consisting of tubes II and '32 and "accompanying resistors and'capacitors, will thus operateasa function of'the output of the push-pull amplifier consisting of tubes 23 and 24. Arise'in the cathode potential-oftube 3| will accompany a lowering of the cathode potential of tube 32. Thus, tube-'32 will conduct f heavilyand tube-3| willfbe cut 'o'fi as in convent-ional-trigger circ'uitaction. *A further increase in the potential of the cathode of tube 3| will be accompanied by a further decrease of the potential of cathode of tube 32. This action will tend to make tube 32 conduct more heavily and will not change the setting of the trigger circuit. Thus it is seen that only a 'change of "polarity of the cathode trigger will reverse the setting of the trigger circuit.

The output of the trigger circuit is applied across potentiometers 36, 31, and 4|. The changing voltage from potentiometer 4|, produced by tube 32, is applied to the grid of tube 45. Similarly, the changing voltage from potentiometer -31, produced by tube 3|, is applied to the grid of tube '41.

Tubes 3| 'an'd32conduct alternately and when the frequency'of tube 53 is being controlled the changes will be quite rapid. When the frequency oftube 53 is not being controlled either the plate of tube 3| or the plate of tube 32 will be at nearly the same potential as the supply voltage. Through 'potentiometers 31 and 4| this voltage is applied to the grid of tubes 41 and 45', :respectively. The potentiometer "whichha's maximum voltage is adjusted so that relay 49 clo'sesreliably. The same is thendone with the trigger circuit reversed so that in eitherpositien the relay will be closed. 1

When controlling the-irequen'cyof tube-53',

'relay 44'to rotate. High and'low points on't-rie cam are made such-that if the motor runs continuously,the heating and cooling time of strut 62 will be sufficiently long that tube 53 will sweep through the desired frequency range. 'The'tifi'gger circuit is sensitive to signals only whenjtl'ie tube 5 3 is sweeping in the desired direction. Fig. 1 shows the cam in position so that-the flipflop is sensitive. Terminal 50 is connected through terminals 55.56, to' potentiometer 36 so that the control voltage of thestrut "Sr-comes *from the tri-gger circuit. If -th'e*cam rotates to the insensitive position term inal '58-wi1l come in contact with terminal 59, causing the grid 0: tube-32 to be grounded. This makes it impossible for pulses to have any effect on the trigger-clifcuit. The grounding of-thegr id'of-tube 3'2-insures that the trigger circuit willbe"-incondition that potentiometer 36 will have'a makimhm-voltage on it when-the trigger circuitis'again matte sensitive by the ungroundingof terminal-58'} groundingof the grid of "tube 3 2, by renderingthe trigger "circuit 3| 32 insensitive to discriminator -23, 2-4; "further insures I that the tube's isenu=-tii twillhold the relay -49 closed and permitthemotor returnedto their normal positions. 'I'lie'rotafor the discriminator 'and' stops motoril when terminal 5 8 "isgrounded, terminal operated so that terminal 50 is connected through axscasoa terminals 56 and-51 to a source of variable voltage. This voltage can be adjusted to any desired value so that when the trigger circuit is insensitive, the frequency of tube 53 will be swept back to the desired value.

I The position of the contact on potentiometer +36 is such that when the maximum voltage is '''applied to potentiometer 36 by the trigger circuit, "the strut 62 will have its maximum power dissipation.

In general, the output voltage is used to control "the current passing through strut 62 mounted with the local oscillator tube. An increase or 5 decrease of the current passing through said strut causes it to expand or contract and thus varies the capacitance of the oscillators cavity thereby changing the frequency of oscillation. Fig. 2, showing discriminator voltage versus local oscillator frequency, illustrates the operating principles of this circuit. Whether the intermediate 1 frequency is too high or too low is indicated by the polarity of the output voltage from the dis- .1 criminator. The design allows for the elimination of either the sum or the difference sideband -*(F1.o=Fsi -F1-F). Since the multivibrator has --two stable conditions, when one plate has been lowered in potential and the other raised, one tube will remain conducting and the other nonconducting until an opposite polarity cathode -trigger occurs. Thus, if the discriminator indicates the I.-F. is too low, the multivibrator will be triggered and will adjust the thermal control .so as to change the local oscillator frequency, and 7 this change continues until the I.-F. is too high, at which time the discriminator will produce a' trigger of opposite polarity. This back and forth type of control is one of the unique aspects of this circuit.

Fig. 2 represents the output of the discriminator as a function of the local-oscillator frequency. The first curve is produced when the local oscillator frequency is approximately equal to the radio frequency minus the intermediate frequency. The curve on the right is produced when the local oscillator frequency isapproximatel equal to the radio frequency plus the intermediate frequency. Assuming that a voltage 9f positive polarity from the discriminator will ,cause the local oscillator frequency to increase, and conversely negative Voltage from the discriminator will cause the local oscillator to decrease, it will .be seen that only one trap, where the frequency is held betwen opposite polarities, existsfor the local oscillator frequency. If it is desirable to have the trap about the right curve" of Fig. 2 instead of the first curve, a reversal of the thermal control at the trigger circuit will produce this effect.

The heating element used to change the frequency has an averaging effect and is thus a long time constant heating device. The short time constant electrical device used in this circuit to control the heating device does not allow an undesirable hunting effect to occur due to the diff erence in time constants.

If the discriminator has not yet gained control to keep the local oscillator within the abovementioned trap, as during the warming-up operation of the apparatus, means are supplied to bring the frequency of the local oscillator under the control of the discriminator. It is also possible, due to abnormal operating conditions for the discriminator to lose control of the local oscillator frequency. Therefore, if the discriminator does .what is at present considered to be a preferred :embodimentof this invention, it will be obvious to those skilled in. the art that various changes and modifications may be made therein without departing from the scope of the invention as set forth in the appended claims,

The invention claimed is:v 1

1. An automatic frequency control systemfor maintaining an input frequency at a desired frequency level, comprising means for varying; said input frequency, ,normally inoperative, control means coupled. to said frequency-varying means for effecting a sweep of said input frequency over a predetermined band of frequencies which includes said desired frequency level, frequency discriminating means for producing anoutput of a first polarity when said input frequency, is within a first small range of frequencies immediatelyabove said desired frequency level and for producing an output of opposite polaritywhen said input frequency is within a second small range of frequencies immediately below said desired frequencylevel, a circuit connectedto said discriminating means having two alternative stable outputs, said circuit assuming a first stable output in response to said first polarity output being applied thereto and assuming a second stable output in response to said oppositepolarity being applied thereto, means for applyingthe output of said circuit to said frequency-varying means for effecting a varition of said input frequency toward said desired frequency levelfrom one direction in response to said first output and a variation of said input frequency, toward said desired frequency level from the opposite direction in response to said second output, and timedelay means coupling said circuit to saidcontrol means for rendering said control means operative in response to said circuit maintaining either one of said two stable outputs for more than a predetermined time, due to said input frequency being outside of said first or second ranges of frequency,- whereby said control means brings said input frequency within said first or second frequency ranges.

2. In combination, an oscillator tube including thermal means for controlling its frequency, a discriminator circuitfor receiving the oscillator frequency said discriminator circuit being tuned to a predetermined frequency to produce an output of one polarity if the oscillator frequency is above said predetermined frequency and of the opposite polarity if the oscillator frequency is below said predetermined frequency, a trigger circuit including first and second electronic discharge devices for receiving the output of said discriminator circuit and arranged so that one of said electronic discharge devices conducts when the output of said discriminator circuit is one of polarity and the other of said electronic discharge devices conducts when the output of said discriminator is of another polarity, means for passing current through said thermal means when one of said electronic discharge devices conducts and for preventing current from passing through said thermal means when the other of said electronic discharge devices conducts.

3. In combination, an oscillator tube including thermal means for controlling its frequency, a,

matoricircuit and:arrangediso thati thedfirst :electronic discharge device ;con'ducts iwhenxthe output of said discriminator circuit is of one polarity -land thexsecond electronic idischarge .idevice .con-

when: the; output ..of said :discr-iminator .ais

through -t-he thermaLmeans in zresponse to conelection ofthe first electronic discharge t device and istopping thei-passage'ofr currentlthroughuthe thermal means in response-ito the conduction .of

'the second -electronio dischargetdevicelthird and' lourth electronicdischarge devices;xmeanswherebytsaidihird -electronic dischargedevice conducts when said first e'lectronic discharge device con- 'iiucts and whereby sa-id fourth electronic -discharge device :conducts when said second elec- '---t10!-iic discharge "device conducts' but rwhereby "either the thirdorfour-th electronic discharge tievice -'-conducts "heavily if eitherntlie -firstorsecelectronic discharge devices conduct continuously for more than a predeterminedperiod of 'time, means whereby when I either said third -'or fourth= electronic dischargedevices conducts ssuificiently, current is *-not "passed -through said thermal deviee for a predetermined-period of timegandafter saidpredetermined-periodoi-time the first" and secondelectronic-discharge devices arepl-aceddrrs-ne-lastate-of conductivity that current does" flow through sa'idthermal-device.

l/An automatic frequency-'ccntrolsystem for aradio' receiver-having a-given interme'diate frefqueney,-*co-mprising a -'-mixer circuit having an 'in-termediate 1 frequency output, a 'local oscillator -"connecte'd-to'said' mixer-havingmeans for varying thefreqnency thereofyafrequencydiscriminator connected 1 to ='said*-mixer, '-'-which is e'fiective over ca -limited frequency range-in the-vicinitv-of said 'glven inter-mediate frequency, forproducing an *output volta'ge hav-ing characteristics which are a flmction of therelationship oftheintermediate frequency output of said mixer arelative toxsaid =l-given intermediatefrequency a circuit connected to 'asaidrdiscriminator which is "arranged to he respons'ive -to .the characteristics of said output yoltage in: a -inanner such that: said:- circuit i-pro- -duces axfirst'stableioutput'when theintermediate or another polarity, means for 2 passing :current -irequencylof saidwmixer' isi'larger; than-:saidiefi'm :intermediate frequency :andproduces'g acsecond stable nutput'iwhenthe intermediate irequency of said. mixer- ;is :smaller than:saidrgivenrintermediate frequency, means for applying thezoutput 40f usai'cl :circuit :to :zthe frequency-varying smeans -iofvasaidroscillator, said frequency-varying means :being arranged sol-that said. first output initiates -aichange infrequency ofsaid oscillator-:i-mone direction and said second output initiates sa change in-the opposite direction' to thereby mainztainnthe local oscillator at the frequency required :,-to-;produce said given intermediate frequenoyrat said mixer output witha minimumamount inoperative control means coupled between :said lcircuit and? said frequency-varying means whieh magnitude of "frequency hunting, and normally is. rendered operative in response to isaidcircirit 2 'maintaining either one ofsaid twostable outputs 0 for more than'a predetermined time,,for;efiectmg a frequency sweep of said oscillatorito bring t-he intermediate frequency output of :said :mixer scribed in claim '4 wherein: said I frequencywarying means is ,a cavity resonator provided with 'a'movable wall, astrut having one'end connected to said movable wall and. its other end-fixe'd,-,:and

the. output 'ofsa-id circuit generates :a current through-at least a portion of said strut and controls the amount of said current-ingresponseato the first and secondroutput. of "said :circuit being differentfrom'each other so as .to "cause: thermal expansionor contraction of said strut, to-thereby adjust the movable wall. of .said cav-ityresonator.

ERIC DURAND. LOWELL M. HOILINGSWORETH.

REFERENCES CITED Theifollowing references are of'record'in the "file of this patent:

UNITED STATES :PATENTS Number Name *Dat'e 1,907,132 Thurston 'Mayl1'2, 15933 12,044,749 Usselman June l'6,*"-1-936 2267;453 "Foster l'Dec.*' 2331941 286,396 Trevor June 16,1942 2,374,810 Fremlin 'May-1,"l 945 -2,384,379 Ingram Sept. 4, -1 95 2,434,294 .Ginzton Jan. 13,1948 1-2;462 856 Ginzton 'Mar. '1, 1-949 $476,311 Learned July -19, 1949 

